This invention is in the field of devices for reducing the amount of oil spilled by oil tankers.
There is a constant risk of spillage of crude oil and refined petroleum products (such as diesel oil or fuel oil) by ships, boats, and barges that travel on oceans, rivers, lakes, and bays. The largest such vessels are crude oil tankers, often called "very large crude carriers" (VLCC's), many of which carry hundreds of thousands of tons of crude oil. They pose threats of catastrophic spills due to groundings, collisions, heavy storms, fires, and other accidents, as evidenced by spills involving tankers such as the Exxon Valdez, the Amoco Cadiz, and the Torrey Canyon. However, large tankers do not pose the only risk; numerous types of smaller vessels such as small tankers and barges also pose oil spill risks.
As used herein, terms such as tanker, oil tanker, ship, boat, and vessel refer to any type of tanker, ship, boat, barge, or other water-borne floating tank vessel having a rigid hull, which carries crude oil or refined petroleum product as cargo. It does not refer to ships which carry diesel or fuel oil in fuel tanks solely for generating their own power or propulsion. It also does not include floating bladders (often called dracones) or barges or other vessels that are designed for temporarily holding oil, such as for temporary storage or during an oil spill cleanup.
Oil and petroleum are used interchangeably herein, to refer to either crude oil (unrefined petroleum), or to refined petroleum products which are conventionally stored in non-pressurized tanks, such as diesel fuel, fuel oil, gasoline, or jet fuel. It does not include liquified propane or liquified petroleum gases, which are sometimes transported by tankers having refrigerated pressurized tanks.
Oil tankers have rigid external walls, usually made of steel plates welded together to form water-tight, oil-tight seams. In a VLCC (usually defined as a tanker having a capacity of about 100,000 tons or more), the oil tanks within the main hull are divided into numerous isolated tanks by means of water-tight walls called bulkheads. Tank compartments which are separated by bulkheads are not in direct fluid communication with each other except by means of piping networks, which are controlled by valves and pumps. Although numerous different arrangements are used (see, e.g., P. M. Kimon et al, "Segregated Ballast VLCC's," Maritime Reporter/Engineering News, Apr. 1, 1973, pp. 12-13), any VLCC will be divided into multiple segments (usually between five and ten) along the length of the hull, by transverse bulkheads. The compartment at the stern or aft (rear) end of the ship contains the engines; the compartment at the bow (forward) end of the ship, which would be most likely to suffer damage if the ship collides with something, usually contains ballast water. The other segments, between the bow compartment and the engine compartment, are tank compartments which carry oil or ballast water.
Each transverse cargo segment is further divided into three main compartments (two outside compartments along the port and starboard sides, and a center compartment which occupies the middle), by longitudinal bulkheads. For example, a five-by-three arrangement would provide fifteen tank compartments, each of which can be independently filled or drained by pumping systems.
In addition to the structural reinforcement provided by transverse and longitudinal bulkheads, the tank compartments of single-hull VLCC's are also reinforced by two additional types of steel reinforcing members or structures. These are called longitudinal stiffeners, and transverse webs.
Longitudinal stiffeners are relatively short plates (usually about 1.5 meters or less in length) which are welded directly to a wall. These stiffeners typically are welded to four different types of walls. callout number 104 in FIG. 5) reinforce the outermost side wall (usually called the side shell), and bottom stiffeners (shown by callout number 106 in FIG. 5) reinforce the bottom wall (the bottom shell). Those are the only stiffeners of direct interest to this invention, although it should be noted that deck stiffeners are also used to reinforce the upper deck, and internal stiffeners are used to reinforce the longitudinal bulkheads, which separate the center tanks from the outer tanks.
Stiffeners mounted on a vertical wall (a side shell or longitudinal bulkhead) typically have L-shaped cross-sections, where the vertical portion points downward, as shown in FIG. 5. This avoids the creation of trapped pools of oil or ballast water in the "bays" between the stiffeners when the tank is drained. The gap or trough area between two adjacent structural members in a tanker is usually referred to as a "bay." A bay between two identical types of members can be called, for example, a stiffener bay (if it is located between two adjacent stiffeners), or a web bay (between two web members; a web bay is much larger and usually includes a number of stiffener bays).
Bottom stiffeners, which stand vertically, usually have T-shaped, L-shaped, or I-shaped cross-sections. "Limber holes" are usually cut through bottom stiffeners, so that oil or water can flow through the holes and pass from one bay to another as a tank is emptied. This avoids the collection of standing puddles in bottom stiffener bays.
In some tankers, stiffeners are interspersed with longitudinal girders. These are comparable in shape but substantially larger than standard stiffeners. Usually, such girders are affixed only to bottom shells, to provide additional support during drydocking.
VLCC's contain two major types of transverse reinforcing members: (1) transverse bulkheads (discussed above), which provide watertight closures that separate different tank compartments, and (2) transverse webs, which do not provide watertight closures. Webs only provide reinforcement; they typically are made of large plates welded together around the internal periphery of a tank, leaving large openings through the center of the web. Each transverse web spans the entire width of the tank compartment it reinforces. Typically, transverse webs are spaced about three to five meters apart from each other along the keel line of a ship. Several such webs are present in each tank, and they are usually spaced identically in the outer and center tanks, so that they will butt up against each other on opposite sides of each longitudinal bulkhead. This provides additional reinforcement against bending forces in the bulkheads. Each longitudinal stiffener or girder normally spans the entire distance between two transverse reinforcing members; accordingly, a stiffener bay is bounded by two stiffeners (along each side of the bay) and two transverse members (webs or bulkheads, at each end of the bay).
Drawings of stiffeners and transverse webs in VLCC's are shown in various books such as The Lore of Ships (Crescent Books, N.Y., 1975) at page 31 and in a pictorial article in the July, 1978 issue of National Geographic.